1. Field of the Invention
This invention relates to tape drive data storage systems. More particularly, the invention is directed to controlling the path of a streaming tape medium relative to the tape drive transducing elements that read and write data on the tape.
2. Description of the Prior Art
By way of background, during the operation of a tape drive data storage apparatus, a tape medium is transferred back and forth between supply and take-up reels while data is read from or written to tape by one or more read/write heads. Typically, the tape medium and the supply reel are mounted inside a tape cartridge that is inserted into a slot in the tape drive. When the tape cartridge is situated in its operational position within the slot, a tape feed mechanism advances the tape onto the take up reel (which is typically within the tape drive) and into registered engagement with the read/write heads. The tape is then advanced past the read/write head(s) by means of a pair of motors, one for each reel, which drive the reels at a desired tape speed.
For optimum data transfer, the tape must be precisely moved through the tape path and across the read/write heads. As such, modern tape drives typically implement tape guides to guide the tape at a desired wrap angle around the heads. FIG. 1 is illustrative. It shows a tape medium “T” spooling in a feed direction “D” over a read/write head assembly “H” between a supply reel “SR” and a take-up reel “TR”. Two primary tape guides, shown as roller bearings (rollers) “R1,” are disposed in close proximity to the head assembly “H,” one on each side thereof. Other tape path guide components, such as secondary rollers “R2,” may also be disposed between the head assembly “H” and the supply and take-up reels “SR” and “TR” to define the tape path.
As can be seen in FIG. 2, data is written by the head assembly “H” onto the tape “T” as a set of parallel tracks that extend longitudinally in the direction of tape movement “D.” The read/write transducers of the head assembly “H” are situated in a central transducing area of the head assembly “H.” This transducer area (labeled “TA”) is shown in FIG. 2 as it transduces several parallel data tracks on the tape “T.” Because the length of the transducing area “TA” is typically substantially less than the width of the tape “T,” the head assembly “H” must be stepped across the tape “T” during drive operation in order to fill all of the available data tracks on the tape with data. This head assembly positioning is performed by a servo actuation system “SAS” that comprises positioning motors for moving the head assembly in a cross-track direction. Servo inputs are typically provided by a pair of servo readers on the head assembly “H” that sense prerecorded servo tracks (not shown) on tape “T.”
For best performance, lateral motion of the tape (transverse to the direction of tape travel) should be minimized because such movement can lead to unreliable positioning of the tape “T” relative to the head assembly read/write transducers. This can produce low readback signal amplitude and poor data transfer reliability. Events that may produce lateral tape motion include (1) tape runout caused by poor stacking on the reels (stack shifts or stagger wraps), wherein one wrap of the tape “T” is substantially laterally offset with respect to adjacent wraps, (2) a buckled tape edge caused by the edge of the tape “T” crawling against a guide roller flange and then shifting laterally back to a normal position, (3) a damaged edge of the tape that causes the tape to flick laterally when contacting a guide roller, and (4) a roller flange that has become scalloped from tape wear, causing the tape to cyclically flick laterally as the tape edge contacts the scalloped area.
Although the tape drive servo actuation system “SAS” is capable of compensating for some lateral tape motion, it cannot handle transient lateral movements that are beyond the servo response capabilities of the system. That is to say, there are some lateral transients that are simply too fast or too large for the servo actuation system “SAS,” such that data read/write errors cannot be avoided. This imposes an artificial limit on data areal densities insofar as data tracks must be sufficient spaced from each other to avoid cross-track overwrites in the event that lateral transients occur during data write operations.
Attempts have been made to constrain lateral tape movement by constructing tape guide rollers with friction enhancing surface properties that limit lateral tape movement by gripping the tape with increased frictional force. Although such solutions have resulted in considerable improvement in lateral tape movement control, it is submitted that additional tape path control may be achieved by considering other guide roller design characteristics.
Accordingly, it is desired to provide an improved design for controlling a tape path in a tape drive data storage system. What is particularly needed is a technique for limiting lateral tape movement by considering the shape, size and alignment of tape path guide rollers as a further solution to preventing tape misalignment problems.